Multiple-mode induction systems for automotive engines have been known for some time. Their use has not been popular for a variety of reasons, including high cost, engineering complexity, and overall inefficiency. However, the general concept is that of providing increased torque at low engine speeds and increased power at higher engine speeds. This is usually accomplished by varying the effective length of the air intake manifold or changing its tuning so that the frequency or pulsations of the air column through the manifold matches the frequency with which the intake valve is opening in the engine cylinder. At low speeds, the intake valves open less frequently per unit time and remain open longer per unit time than at higher speeds. Thus, a longer flow pipe or manifold is desirable at low speeds and a shorter flow pipe or manifold at higher speeds. Matching the frequency of air flow to valve opening provides a ram effect or more complete filling of the combustion chamber with air for combustion.
One such system is shown in U.S. Pat. No. 2,894,497 to include a ram tube type manifold for each cylinder wherein the effective length can be infinitely varied over a predetermined range. This maximizes the ram effect of the incoming intake air to the combustion chamber connected with the manifold. In turn, increased volumetric efficiency within the cylinders is obtainable throughout a selected speed range of the engine. Not only is the manifold length adjusted to provide a change in tuning or resonant frequency, but also the intake air feed is maximized because there is direct feed through from the air inlet through the effective length of the manifold.
However, this general type system has not been put into widespread use. Its expense and complexity including its many components and moving parts makes it unsuitable for everyday production vehicles.
Instead, industry has shown most acceptance for simpler dual-mode induction systems wherein the runner type intake manifold has in effect two lengths, i.e. a long runner optimized in length for low speed ranges and a short runner optimized in length for the higher speed ranges. Such systems seem to offer the most advantage concerning relative cost for performance. They generally include a common air inlet to a single air distribution chamber with manifold runners extending therebetween to each respective engine cylinder. Most of these, however, seem to be specifically adapted for in-line engines. Moreover, they provide a change in tuning, and generally do nothing to change the air feed through the system, as shown for example in U.S. Pat. Nos. 4,664,076 to Miyano et al; 4,679,531 to Hitomi; 4,727,829 to Kondo; 4,765,285 to Kobayashi; and 4,771,740 to Koike.
For V-type engines, U.S. Pat. No. 4,669,428 to Ichida discloses a dual-mode intake manifold designed for such use. It provides a dual effective length induction tube having dual resonant frequencies and corresponding dual air feed paths. The change from one effective length to the other is made by opening or closing a butterfly valve located in each respective induction tube. While the operational concept appears to have been reduced to its simplest form, the manner of closing off the long flow path by providing individual control valves in each flow tube is considered undesirable. Further, it will be noted that the intake manifold runners extend generally parallel to the engine longitudinal axis and includes a large plenum in front of the engine generally perpendicular to the crankshaft axis. Such a layout makes it particularly difficult to package or install the system in the ever increasingly popular front-wheel drive vehicle having a transversely mounted engine.